1. Field of the Invention
The present invention generally relates to lighting devices and image sensors, and more particularly to a lighting device for an image sensor employed in an original reading device and an image sensor using the same.
2. Description of the Related Art
Image forming apparatuses that can read an original, such as copiers, facsimile machines, and scanners, employ a lighting device including a light source for reading.
Some lighting devices include, as an image sensor, a line sensor provided with a light source such as a fluorescent lamp or a halogen lamp running parallel to the main scanning direction of an original.
In a reading device employed in image forming apparatuses, the relationship between electric potential contrast (illuminance) on its latent image carrier and the amount of toner attached thereto is represented by the gamma characteristic, which affects the difference in density between an original and its copy. In the gamma characteristic, unevenness in illuminance also causes unevenness in the amount of toner attached, thereby adversely affecting the sharpness of the copied image. In order to solve this problem, Japanese Laid-Open Patent Applications No. 5-199366 and No. 7-319082 have proposed configurations for reducing unevenness in illuminance in a lighting device employed in a reading device.
According to the disclosure of each of the above-described patent applications, an auxiliary reflector for directing illuminating light emitted from a light source is provided in a position opposite the light source, and a shield member is provided between the light source and the surface of an original so that the quantity of light is uniformly distributed in the main scanning direction.
Normally, an original to be illuminated is scanned by receiving illuminating light from a lighting device while being kept flat on an original placement face. However, such originals as are undulated by wrinkles or stepped are also subjected to scanning.
Generally, in the case of illuminating a wrinkled or stepped original with only illuminating light from a light source, shade may be formed in the illuminated part of the original due to the singularity of the light source. As a result of the existence of the shade, information different from the actual original information is obtained as read information. In order to eliminate this disadvantage, two configurations have been proposed. That is, a plurality of light sources are provided, or an auxiliary reflector is provided as disclosed in the above-mentioned patent applications.
Of these two configurations, the latter is preferable to the former because the former has a disadvantage of requiring a space for the light sources to occupy in an apparatus in which the light sources are provided, which increases the apparatus in size. The latter, however, has the following new problem.
In the case of providing the auxiliary reflector, unevenness in lighting can be reduced by combining light emitted from a light source to directly illuminate the surface of an original with auxiliary light reflected back from the auxiliary reflector. However, the direct light from the light source and the auxiliary light from the auxiliary reflector are not balanced in quantity. Particularly, the auxiliary light is less in quantity than the direct light. Therefore, in the case of scanning a wrinkled or stepped original, the shade generated therein may not be eliminated depending on the intensity of the direct light. Further, in the case of providing the auxiliary reflector, it may be difficult to adjust its strength of reflection depending on the setting of its arrangement, thus making it difficult to obtain a balance in the quantities of light required for eliminating the shade.